OS33D-01 13:40h
Northwest Australian Shelf Dynamics Experiment: Along-Shelf Flow
During June-July 2003, shelf and slope physical processes off northwestern Australia were the focus of a one-month cruise. The experiment was designed to study the effects of evaporative densification over the shelf (see Shearman et al. abstract) and the relation of shelf edge currents to the poleward Leeuwin current. Sampling approaches included a broad-scale CTD survey, SeaSoar profiling, MiniBat profiling in shallower waters, underway acoustic current profiling and deployment of a shelf-edge current meter mooring. Shelf waters were found to be saltier than those offshore, and fresher water was found to the northeast. There was usually a shelf edge jet, although the direction varied. When it was poleward, it resembled the Leeuwin Current in magnitude and relative freshness. Repeated three-dimensional synoptic sampling near the shelf edge took place while an equatorward, bottom-trapped jet dominated at the shelf edge. The jet meandered with time, and so we suspect that the basic equatorward flow was unstable. The shelf edge processes in this area were unusual in that a bottom-trapped jet was found, but it appears that the upstream extension of the Leeuwin Current was observed at least on occasion
OS33D-02 13:55h
Northwest Australian Shelf Dynamics Experiment: Shelf-Edge Processes
Repeated, well-resolved cross-shelf sections of temperature, salinity and currents were made near the northwestern Australian shelf edge during June and July, 2003, using a towed undulating instrument platform (SeaSoar) and a ship-based acoustic Doppler current profiler (ADCP). The shelf waters are typified by strong semidiurnal tides and by intense evaporative densification. When the shelf-edge alongshore flow was poleward, saltier waters were found only over the shelf, but did not extend offshore of the shelfbreak. Most of the sampling occurred while there was a bottom-trapped equatorward flow along the shelf edge, and there was invariably a tongue of salty water protruding offshore of the shelf edge into the ambient fresher water offshore. This tongue was typically centered at about 125m depth, and was about 50m thick. The salty tongue's origin appears to be related to bottom Ekman transport convergence (due to cross-shelf variations in alongshore flow). Although the salty tongue was always present when there was an equatorward shelf-edge flow, it was subject to considerable temporal variability that may be associated with linear or nonlinear internal tides generated at the shelf break. It thus appears that shelf-slope exchanges may be strongly influenced by the sense of alongshore flow.
OS33D-03 14:10h
Northwest Australian Shelf Dynamics Experiment: Evaporative Dense Water Formation over the Inner-Shelf
During the June-July 2003 Northwest Australian Shelf Dynamics Experiment, several high-resolution along- and cross-shelf sections of temperature, salinity, and currents were collected over the inner-shelf (20-50 m water depth), using a towed undulating instrument platform (MiniBat) and shipboard acoustic Doppler current profiler (ADCP). Additional ship-based sensors observed air temperature, sea surface temperature, relative humidity, precipitation, downward long and short wave radiation, and wind speed and direction - from these observations surface heat and freshwater fluxes were estimated. The inner-shelf is characterized by increasingly cool, salty and dense waters onshore, and a strong front is present near the 25 m isobath. Vertically, inner-shelf waters are characterized by a dramatic two-layer system; a well-mixed surface layer that shares characteristics with the offshore surface mixed layer and a cool, salty bottom layer that extends from the inner-shelf front, across the shelf, and detaches along the main thermocline at the shelf break. Analysis of the ship-based meteorological measurements reveal a large latent heat flux out of the ocean that overwhelms insolation and maintains the well-mixed surface layer and the cross-shelf structure (increasingly cool, salty and dense water onshore).
OS33D-04 14:25h
The dynamics and cross-frontal circulation at a tidal mixing front under well-mixed and stratified conditions
We use current, temperature, salinity and bottom pressure data from an array of moorings deployed on the southern flank of Georges Bank to estimate the cross-bank momentum budget and the tidal and sub-tidal circulation in the vicinity of the tidal mixing front (TMF) that is often present in that region. The array of eight moorings was deployed for a 160 day period during the spring and summer of 1999, spanning approximately 20 km in the cross-bank direction and centered on the 60 m isobath, the typical location of the TMF on the southern flank. The tight spacing between moorings (2 to 3 km), allowed us to determine the dominant balance in the sub-tidal, cross-front momentum equation and the detailed structure of the tidal and sub-tidal cross-front and along-front circulation during the spring, when the water column was well-mixed and the TMF was not present, and during the summer, when a strong thermal TMF was observed. Notably, we directly calculate the cross-frontal tidal stress terms associated with tidal rectification. During well-mixed conditions with weak winds, the cross-frontal tidal stress is uniform with depth and approximately balances the Coriolis term associated with the along-bank circulation. When a strong thermal tidal mixing front is present during weak winds, tidal stress, cross-front baroclinic pressure gradient and Coriolis are dominant terms in the cross-front momentum budget. The different dynamical balances between spring and summer conditions lead to differences in the cross-frontal and along-frontal circulation. During the spring, the along-bank, sub-tidal circulation is clockwise (to the southwest), uniform with depth, with an amplitude of about 8 cm/s. The cross-front circulation is mode-one in structure, with onbank flow near the bottom and offbank flow near the surface, and with an amplitude of about 2 cm/s. During the summer, a strong subsurface along-bank jet with an amplitude of 30 cm/s was observed at the TMF. The sub-tidal, cross-front circulation is stronger and more complicated than during the spring. At the location of the jet, the cross-front flow is on-bank in the middle of the water column, with an amplitude of 5 cm/s. The flow is offbank near the surface and bottom of the water column. We explore the implications of the variations in the cross-frontal circulation to the cross-frontal heat flux and the heat budget.
OS33D-05 14:40h
A Sulfur Hexafluoride-Based Lagrangian Study on Initiation and Accumulation of the Red Tide {\it Cochlodinium} in Southern Coastal Waters of Korea
During the last two decades, massive accumulations of autotrophic algae and some heterotrophic protists, collectively referred to as harmful algal blooms (HABs), have increased considerably in frequency, size, and cell density across the globe. Several field studies suggest increasing eutrophication as the primary reason for the increases in the blooms; however, strong evidence for this relation is not yet available, largely due to the ephemeral and complex nature of HABs. Here we report the first continuous in situ measurements of a population of the HAB species {\it C. polykrikoides} in a fixed volume of inshore waters near the island of Naro-do (34.47$\deg$N and 127.55$\deg$W), an area that lies off the southern coast of Korea and where the first bloom of {\it C. polykrikoides} has occurred at least for each of the past five years. This Lagrangian experiment was carried out by injecting the inert chemical tracer sulfur hexafluoride (SF$_{6}$) into a patch of seawater carrying {\it C. polykrikoides} and tracking the SF$_{6}$-labeled water mass for 4 days. Our results suggest that bloom initiation and much of the cell accumulation in the early phase of the bloom are due to the input of {\it C. polykrikoides} cells from a shoreward movement of alongshore currents.
OS33D-06 14:55h
Observations of onshore sediment transport in water depths of 10-20 m off Long Island, NY
A high-resolution multibeam echosounder has been used as part of investigations of sediment transport, benthic habitat and coastal infrastructure in a number of estuarine and inner shelf settings. High-resolution multibeam topographic data provides important information on the nature and distribution of sedimentary bed forms while co-registered backscatter at 300 kHz provides insights into sediment type and the presence of smaller bed forms. A number of studies have been conducted on the inner continental shelf off Long Island, New York to characterize surf-clam habitat and the present-day structure of artificial reefs. These studies, which range in depth from to over 20 m, often show the existence of asymmetric sand waves and other features caused by sediment movement (especially scour depressions and backscatter anomalies) which indicate onshore sediment transport. Repeat surveys show that sand waves can migrate onshore as much 15 m over a 6-month period, although migration is likely to be episodic, occurring primarily during storms or other events. This high-resolution bathymetric data shows the sand waves are not uniformly distributed across the sea bed, but are often concentrated on the flanks of elongate topographic depressions. This onshore sediment movement appears to be occurring along the length of Long Island, although there are regional variations in the water depth in which they are observed. This observed transport may represent a portion of the onshore sediment transport thought to exist in this area.
OS33D-07 15:10h
Space variability and time evolution of water quality parameters in coastal area. Results from the lagoon of Venice case study
The paper presents the result of a number of numerical analysis on nutrients dynamic and plankton evolution in the lagoon of Venice. First we present the results of a statistical analysis on water quality parameters measured in the last 3 years in the Lagoon of Venice, with the aim to draw a basin-scale picture of the spatial distribution and seasonal evolution of macronutrients and chlorophyll a, which is here considered as a proxy of the abundance of the phytoplankton. Multivariate technique and gridding methods were used for investigating the spatial and seasonal variability of the data and for characterizing the trophic evolution of the basin in 2001. The results of the multivariate analysis indicate that the thirty stations can be classified into seven groups, based on the similarity of the temporal evolution of nutrient and clorophill a. Furthermore, these groups reflect the geographic distribution of the station, since the stations closer to the tributary discharges and the ones which are located in southern part of the lagoon were classified into clearly distinct cluster. The analysis of the temporal evolutions of the spatial distributions of the most important water quality parameters, confirms such an interpretation and evidences the role of the tributaries and of the exchanges with the sea in determining the water quality and the dynamics of the ecosystem. The comparison among the three years offers additional support to this interpretation. Secondly we propose a partition of the lagoon in different sub-basins, on the basis of spatial distribution of physical properties, such as relative composition of water entering the lagoon from each of the three inlets, residence time, ventilation time, average salinity, bathymetry. Such a physical based partition is then compared to the classification which emerges from analysis of water quality parameters, and a discussion on the role of the physical and environmental factors which affect such variability is offered. Thirdly, the key role of physical factors is clearly demonstrated and established in quantitative terms by the results of the regression analysis between a trophic indicator, identified in the multivariate analysis, and the yearly averaged values of salinity and residence time, that is of physical parameters which give a measure of the influence of the rivers and of the tidal mixing. A comparison among different yeasr Finally, deterministic models are used to complement the above mentioned findings, by providing estimates of fluxes among macro-ecological compartments, and assessment of the response of the lagoon to different scenario of antrophic forcing and meteo-climatic conditions.
OS33D-08 15:25h
The Effect of Hurricane Lili on the Distribution of Organic Matter in the Inner Louisiana Shelf (Gulf of Mexico, USA)
Suspended particles and surface sediments were collected from the inner shelf of the Louisiana central coast following the passage of Hurricane Lili. The elemental and stable isotopic data of these samples were compared to those determined prior to the hurricane. A week after the storm passage, turbidity levels and total suspended sediment concentrations were not that different from pre-storm values, suggesting most of the suspended sediments must have settled soon after the storm passage. Particulate organic carbon (POC) concentrations ranged from 0.1 mg/L to over 2.0 mg/L, with the highest concentrations measured near the seabed and in the inshore portions of the study area. In these locations, suspended particles displayed high POC/Chlorophyll ratios (POC/Chl of 400 to 4,000) and relatively elevated POC/particulate nitrogen ratios (POC/PN of 10 to 14) that indicated their source was locally resuspended seabed sediments. Relatively low POC/Chl (~200) and POC:PN (~7) ratios consistent with significant contributions from phytoplankton were measured only in the surface waters of the most offshore locations. Post hurricane sediment deposition resulted in a storm layer that ranged from 0.5 to 19 cm in thickness. The storm layer was generally composed of silty clays with a coarser, somewhat sandy 1-2 cm basal layer. These storm deposits were characterized by relatively high SA and OC contents. Similarities in the characteristics of the organic matter before and after reinforce the hypothesis that the source of the storm deposits was the finer fraction of locally resuspended seabed sediments, with little evidence for allochthonous land-derived inputs.